Polyol ester lubricants for hermetically sealed refrigerating compressors

ABSTRACT

A high quality lubricant for hermetically sealed domestic air conditioner and refrigerator compressors, especially those using chlorine free hydrofluorocarbon refrigerant working fluids, is provided by mixed esters of hindered polyols, most desirably pentaerythritol, with a mixture of carboxylic acids including at least some iso-pentanoic acid along with either or both of iso-nonanoic acid and dibasic acids such as adipic.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of copendingInternational Application No. PCT/US92/04438 designating the UnitedStates and filed Jun. 3, 1992, the entire disclosure of which, except tothe extent contrary to any explicit statement herein, is herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to lubricant base stocks, which can alsoserve as complete lubricants in some cases; compounded lubricants, whichinclude at least one additive for such purposes as improving highpressure resistance, corrosion inhibition, and the like along with thelubricant base stocks which contribute the primary lubricity to thecompounded lubricants; refrigerant working fluids including lubricantsaccording to the invention along with primary heat transfer fluids, andmethods for using these materials. The lubricants and lubricant basestocks are generally suitable for use with most or all halocarbonrefrigerants and are particularly suitable for use with substantiallychlorine-free, fluoro-group-containing organic refrigerating heattransfer fluids such as pentafluoroethane, 1,1-difluoroethane,1,1,1-trifluroethane, and tetrafluoroethanes, most particularly1,1,1,2-tetrafluoroethane. The lubricants and base stocks, incombination with these heat transfer fluids, are particularly suitablefor hermetically sealed compressors for domestic air conditioners andrefrigerators, where long lubricant service lifetimes are importantbecause of the difficulty and expense of supplying additional lubricantafter the initial assembly of the compressor.

[0004] 2. Statement of Related Art

[0005] Chlorine-free heat transfer fluids are desirable for use inrefrigerant systems, because their escape into the atmosphere causesless damage to the environment than the currently most commonly usedchlorofluorocarbon heat transfer fluids such as trichlorofluoromethaneand dichlorodifluoromethane. The widespread commercial use ofchlorine-free refrigerant heat transfer fluids has been hindered,however, by the lack of commercially adequate lubricants. This isparticularly true for one of the most desirable working fluids,1,1,1,2-tetrafluoroethane, commonly known in the art as “Refrigerant134a” or simply “R134a”. Other fluoro-substituted ethanes are alsodesirable working fluids.

[0006] Esters of hindered polyols, which are defined for this purpose asorganic molecules containing at least five carbon atoms, at least 2 —OHgroups, and no hydrogen atoms on any carbon atom directly attached to acarbon atom bearing an —OH group, have already been recognized in theart as high quality lubricant basestocks for almost any type ofrefrigeration machinery employing a fluorocarbon refrigerant,particularly one free from chlorine. The following patents and publishedpatent applications also teach many general classes and specificexamples of polyol esters useful as refrigerant lubricants withchlorine-free fluoro group containing heat transfer fluids: U.S. Pat.No. 4,851,144; UK 2 216 541; U.S. Pat. Nos. 5,021,179; 5,096,606; WO90/12849 (Lubrizol); EP 0 406 479 (Kyodo Oil); EP 0 430 657 (Asahi DenkaKK); EP 0 435 253 (Nippon Oil); EP 0 445 610 and 0 445 611 (Hoechst AG);EP 0449 406; EP 0 458 584 (Unichema Chemie BV); and EP 0 485 979(Hitachi).

DESCRIPTION OF THE INVENTION

[0007] Except in the claims and the operating examples, or whereotherwise expressly indicated, all numerical quantities in thisdescription indicating amounts of material or conditions of reactionand/or use are to be understood as modified by the term “about” indefining the broadest scope of the invention. Practice of the inventionwithin the boundaries corresponding to the exact quantities stated isusually preferable, however.

[0008] More specifically, esters according to this invention should havea viscosity of not more than 44, or with increasing preference in theorder given, not more than 42, 40, 38.5, 37.2, 36.6, 36.2, 35.7, or35.2, centistokes at 40° C. Independently, esters according to thisinvention should have a viscosity of at least 22.5, or with increasingpreference in the order given, at least 23.9, 25.0, 25.9, 26.7, 27.4,28.0, 28.5, 29.0, 29.4, or 29.8, centistokes at 40° C.

[0009] It has now been found that selected polyol esters provide highquality lubrication for this kind of service. Specifically effective areesters or mixtures of esters made by reacting (i) a mixture of alcoholmolecules selected from the group consisting of 2,2-dimethylol-1-butanol(also known as “trimethylolpropane” and often abbreviated hereinafter as“TMP”); di-trimethylolpropane (often abbreviated hereinafter as “DTMP”),a molecule with four hydroxyl groups and one ether linkage, formallyderived from two molecules of TMP by removing one hydroxyl group fromone of the TMP molecules and one hydrogen atom from a hydroxyl group ofthe other TMP molecule to form water and join the two remainders of theoriginal TMP molecules with an ether bond;2,2-dimethylol-1,3-propanediol (also known as “pentaerythritol” andoften abbreviated hereinafter as “PE”); and di-pentaerythritol (oftenabbreviated hereinafter as “DPE”), a molecule with six hydroxyl groupsand one ether bond, formally derived from two PE molecules by the sameelimination of the elements of water as described above for DTMP, with(ii) a mixture of acid molecules selected from the group consisting ofall the straight and branched chain monobasic and dibasic carboxylicacids with from four to twelve carbon atoms each, with the alcoholmoieties and acyl groups in the mixture of esters selected subject tothe constraints that (a) a total of at least 3%, or, with increasingpreference in the order given, at least 7, 10, 14, 16, or 19%, of theacyl groups in the mixture 2-methylbutanoyl or 3-methylbutanoyl groups,which are jointly abbreviated hereinafter as “acyl groups from i-C₅acid”; (b) the ratio of the % of acyl groups in the mixture that contain8 or more carbon atoms and are unbranched to the % of acyl groups in themixture that are both branched and contain not more than six, preferablynot more than five, carbon atoms is not greater than 1.56, morepreferably not greater than 1.21, or still more preferably not greaterthan 1.00; (c) the % of acyl groups in the ester mixture that contain atleast nine carbon atoms, whether branched or not, is not greater than81, or increasingly more preferably, not greater than 67 or 49; and (d)not more than 2, more preferably not more than 1, or still morepreferably not more than 0.4, % of the acyl groups in the ester mixtureare from acid molecules with more than two carboxyl groups each; andeither (d)(1) a total of at least 20, or, with increasing preference inthe order given, at least 29, 35, or 41% of the acyl groups in themixture are from one of the trimethylhexanoic acids, most preferablyfrom 3,5,5-trimethylhexanoic acid; and not more than 7.5, or, withincreasing preference in the order given, not more than 6.0, 4.5, 3.0,1.7, 0.9, or 0.4% of the acyl groups in the acid mixture are fromdibasic acids; or (d)(2) at least 2.0, or with increasing preference inthe order given, at least 2.8, 3.6, 4.1, or 4.9, %, but not more than13%, preferably not more than 10%, or still more preferably not morethan 7.0%, of the acyl groups in the ester mixture are from dibasic acidmolecules; and a total of at least 82, or with increasing preference inthe order given, at least 85, 89, 93, 96, or 99% of the monobasic acylgroups in the acid mixture have either five or six, or more preferablyexactly five, carbon atoms each. In all these percentages, acyl groupsare counted as a single group, irrespective of the number of valencesthey have. For example, each molecule of adipic acid yields a single,dibasic, acyl group when completely esterified.

[0010] (Of course, for all the types of esters described herein as partof the invention, it is possible to obtain the same esters or mixture ofesters by reacting acid derivatives such as acid anhydrides, acylchlorides, and esters of the acids with lower molecular weight alcoholsthan those desired in the ester products according to this invention,instead of reacting the acids themselves. The acids are generallypreferred for economy and are normally specified herein, but it is to beunderstood that the esters defined herein by reaction with acids can beequally well obtained by reaction of alcohols with the correspondingacid derivatives, or even by other reactions. The only, critical featureis the mixture of acyl groups and alcohol moieties in the final mixtureof esters formed.)

[0011] Preferably, with increasing preference in the order given, atleast 60, 75, 85, 90, 95, or 98% of the hydroxyl groups in the mixtureof alcohols reacted to make esters according to this invention aremoieties of PE molecules. Independently, in the mixtures reacted to makethe esters according to this invention, with increasing preference inthe order given, at least 60, 75, 85, 90, 95, or 98% of the monobasicacid molecules in the acid mixture consist of molecules having no morethan ten carbon atoms each and, with increasing preference in the ordergiven, at least 60, 75, 85, 90, 95, or 98% of the dibasic acid moleculesin the acid mixture consist of molecules having no more than ten carbonatoms each, or more preferably from five to seven carbon atoms each.Most preferably, with increasing preference in the order given, at least60, 75, 85, 90, 95, or 98% of the monobasic acid molecules in the acidmixture consist of molecules having either five or nine carbon atomseach.

[0012] These preferences for the acyl groups and alcohol moieties inesters according to this invention are based on empirically determinedgeneralizations. In order to achieve the desired middle range ofviscosity, corresponding approximately to ISO grades 22-46, it isadvantageous to have a substantial fraction of alcohols with at leastfour hydroxyl groups. Among the commercially available hindered alcoholsthat satisfy this criterion, PE is less expensive than DTMP and is freefrom the ether linkage in DTMP, which increases the hygroscopicity ofthe esters formed and thereby may promote undesirable corrosion of themetal surfaces lubricated. Alcohols with more than four hydroxyl groupsproduce esters with higher than optimum viscosities, but some suchesters can be tolerated, and mixtures including them may be cheaper.Commercial grade PE often contains a substantial amount of DPE, andcosts at least a little less than more purified PE. When cost factorsare not severely constraining, removing most or all of the DPE from apredominantly PE mixture of alcohols used to make the esters ispreferable, in order to minimize the chance of insolubility of part ofthe ester mixture at low temperatures.

[0013] In order to obtain esters with adequate viscosity, a considerablefraction of the acid molecules reacted need to have eight or more carbonatoms or be dibasic. Dibasic acids are less desirable. They must beused, if at all, in rather small amounts in order to avoid excessiveviscosity, because of the capability of forming very high molecularweight and very viscous oligomers or polymers by reaction betweenalcohol and acid molecules that both have at least two functionalgroups. In practice, it has been found that the amount of dibasic acidthat can be effectively used in the acid mixture reacted to make estersaccording to this invention is substantially less than the amount thatwould be sufficient to provide at least one dibasic acid group to linkeach two alcohol molecules in the alcohol mixture also reacted.Therefore, when such amounts of dibasic acid are used, some of thealcohol molecules will be joined together in the esters formed and somewill not; the esters with two or more alcohol moieties will be much moreviscous and normally less readily soluble in the fluorocarbonrefrigerant fluids than the other esters in the mixture, those with onlyone alcohol moiety, thereby increasing the risk of undesirable phaseseparation in the course of use of the esters. However, limited amountsof dibasic acid may nevertheless be used, as already noted above.

[0014] When substantially only monobasic acids are used to make theesters, as already noted, in order to obtain adequate viscosity in themixture, a substantial fraction of the acid molecules must have at leasteight carbon atoms. With acids of such length, solubility in thefluorocarbon refrigerant fluids is less than for esters with shorteracids, and this reduced solubility is particularly marked for straightchain acids, so that a substantial fraction of the longer acids normallyneeds to be branched; alternatively, it has been found that these longerstraight chain acids can be “balanced” for solubility with an equal ornot too much less than equal fraction of branched acids with five or sixcarbon atoms. When the number of carbon atoms per molecule is nine ormore, not even branching is sufficient to produce adequate solubility byitself, so that an upper limit on the fraction of such acids isindependently required. In general, a minimum amount of the particularlyadvantageous i-C₅ acid is specified to aid in solubilizing the parts ofthe esters in the mixture that contain dibasic acids or those with eightor more carbon atoms.

[0015] For both performance and economic reasons, it has been found thatfive and nine carbon monobasic acids are the most preferredconstituents, and they are very effective in balancing each other toachieve a mix of viscosity and solubility characteristics that is bettersuited than others to most applications. Trimethylhexanoic acids, withtheir three methyl branches, produce the most soluble esters among thereadily available nine carbon acids. (In general, methyl branches arethe most effective in promoting solubility without increasing viscosityexcessively, because of the larger number of carbon atoms in otherbranching groups.) Branches on the carbon alpha to the carboxyl increasethe difficulty of esterification and do not appear to be any moreeffective in increasing solubility than more remotely located branches.The most economical commercially available mixture of branched ninecarbon acids, which contains from 88-95 mole % of3,5,5-trimethylhexanoic acid with all but at most 1 mole % of theremainder being other branched C₉ monobasic acids, appears at least aseffective as any other and is therefore preferred for economic reasonsas the source of C₉ monobasic acids.

[0016] It is to be understood that only the desired alcohols and acidsare explicitly specified, but some amount of the sort of impuritiesnormally present in commercial or industrial grade products can betolerated in most cases. For example, commercial pentaerythritolnormally contains only about 85-90 mole % of pure pentaerythritol, alongwith 10-15 mole t of di-pentaerythritol, and commercial pentaerythritolis satisfactory for use in making lubricant esters according to thisinvention in many cases. In general, however, it is preferred, withincreasing preference in the order given, that not more than 25, 21, 17,12, 8, 5, 3, 2, 1, 0.5, or 0.2% of either the hydroxyl groups in thealcohol mixtures specified herein or of the carboxyl groups in the acidmixtures specified herein should be part of any molecules other thanthose explicitly specified for each type of lubricant base stock.Percentages of specific chemical molecules or moieties specified herein,such as the percentages of carboxyl and hydroxyl groups stated in thepreceding sentence, are to be understood as number percentages, whichwill be mathematically identical to percentages by chemical equivalents,with Avogadro's number of each specified chemical moiety regarded as asingle chemical equivalent.

[0017] The above descriptions for each of the acid and alcohol mixturesreacted to produce lubricant esters according to this invention refersonly to the mixture of acids or alcohols that actually reacts to formesters and does not necessarily imply that the mixtures of acids oralcohols contacted with each other for the purpose of reaction will havethe same composition as the mixture that actually reacts. In fact, ithas been found that reaction between the alcohol(s) and the acid(s) usedproceeds more effectively if the quantity of acid charged to thereaction mixture initially is enough to provide an excess of 10-25% ofequivalents of acid over the equivalents of alcohol reacted with theacid. (An equivalent of acid is defined for the purposes of thisspecification as the amount containing one gram equivalent weight ofcarboxyl groups, while an equivalent of alcohol is the amount containingone gram equivalent weight of hydroxyl groups.) The composition of themixture of acids that actually reacted can be determined by analysis ofthe product ester mixture for its acyl group content.

[0018] In making most or all of the esters and mixtures of esterspreferred according to this invention, the acid(s) reacted will be lowerboiling than the alcohol(s) reacted and the product ester(s). When thiscondition obtains, it is preferred to remove the bulk of any excess acidremaining at the end of the esterification reaction by distillation,most preferably at a low pressure such as 1-5 torr.

[0019] After such vacuum distillation, the product is often ready foruse as a lubricant or lubricant base stock according to this invention.If further refinement of the product is desired, the content of freeacid in the product after the first vacuum distillation may be furtherreduced by treatment with epoxy esters as taught in U.S. Pat. No.3,485,754 or by neutralization with any suitable alkaline material suchas lime, alkali metal hydroxide, or alkali metal carbonates. Iftreatment with epoxy esters is used, excess epoxy ester may be removedby a second distillation under very low pressure, while the products ofreaction between the epoxy ester and residual acid may be left behind inthe product without harm. If neutralization with alkali is used as therefinement method, subsequent washing with water, to remove anyunreacted excess alkali and the small amount of soap formed from theexcess fatty acid neutralized by the alkali, is strongly preferredbefore using the product as a lubricant and/or base stock according tothis invention.

[0020] Under some conditions of use, the ester(s) as described hereinwill function satisfactorily as complete lubricants. It is generallypreferable, however, for a complete lubricant to contain other materialsgenerally denoted in the art as additives, such as oxidation resistanceand thermal stability improvers, corrosion inhibitors, metaldeactivators, lubricity additives, viscosity index improvers, pourand/or floc point depressants, detergents, dispersants, antifoamingagents, anti-wear agents, and extreme pressure resistant additives. Manyadditives are multifunctional. For example, certain additives may impartboth anti-wear and extreme pressure resistance properties, or functionboth as a metal deactivator and a corrosion inhibitor. Cumulatively, alladditives preferably do not exceed 8% by weight, or more preferably donot exceed 5% by weight, of the total compounded lubricant formulation.

[0021] An effective amount of the foregoing additive types is generallyin the range from 0.01 to 5% for the anti-oxidant component, 0.01 to 5%for the corrosion inhibitor component, from 0.001 to 0.5% for the metaldeactivator component, from 0.5 to 5% for the lubricity additives, from0.01 to 2% for each of the viscosity index improvers and pour and/orfloc point depressants, from 0.1 to 5% for each of the detergents anddispersants, from 0.001 to 0.1% for anti-foam agents, and from 0.1-2%for each of the anti-wear and extreme pressure resistance components.All these percentages are by weight and are based on the total lubricantcomposition. It is to be understood that more or less than the statedamounts of additives may be more suitable to particular circumstances,and that a single molecular type or a mixture of types may be used foreach type of additive component. Also, the examples listed below areintended to be merely illustrative and not limiting, except as describedin the appended claims.

[0022] Examples of suitable oxidation resistance and thermal stabilityimprovers are diphenyl-, dinaphthyl-, and phenylnaphthyl-amines, inwhich the phenyl and naphthyl groups can be substituted, e.g.,N,N′-diphenyl phenylenediamine, p-octyldiphenylamine,p,p-dioctyldiphenylamine, N-phenyl-1-naphthyl amine, N-phenyl-2-naphthylamine, N-(p-dodecyl)phenyl-2-naphthyl amine, di-1-naphthylamine, anddi-2-naphthylamine; phenothazines such as N-alkylphenothiazines;imino(bisbenzyl); and hindered phenols such as 6-(t-butyl) phenol,2,6-di-(t-butyl) phenol, 4-methyl-2,6-di-(t-butyl) phenol,4,4′-methylenebis(-2,6-di-{t-butyl} phenol), and the like.

[0023] Examples of suitable cuprous metal deactivators are imidazole,benzamidazole, 2-mercaptobenzthiazole, 2,5-di-mercaptothiadiazole,salicylidine-propylenediamine, pyrazole, benzotriazole, tolutriazole,2-methylbenzamidazole, 3,5-dimethyl pyrazole, and methylenebis-benzotriazole. Benzotriazole derivatives are preferred. Otherexamples of more general metal deactivators and/or corrosion inhibitorsinclude organic acids and their esters, metal salts, and anhydrides,e.g., N-oleyl-sarcosine, sorbitan monooleate, lead naphthenate,dodecenyl-succinic acid and its partial esters and amides, and4-nonylphenoxy acetic acid; primary, secondary, and tertiary aliphaticand cycloaliphatic amines and amine salts of organic and inorganicacids, e.g., oil-soluble alkylammonium carboxylates; heterocyclicnitrogen containing compounds, e.g., thiadiazoles, substitutedimidazolines, and oxazolines; quinolines, quinones, and anthraquinones;propyl gallate; barium dinonyl naphthalene sulfonate; ester and amidederivatives of alkenyl succinic anhydrides or acids, dithiocarbamates,dithiophosphates; amine salts of alkyl acid phosphates and theirderivatives.

[0024] Examples of suitable lubricity additives include long chainderivatives of fatty acids and natural oils, such as esters, amines,amides, imidazolines, and borates.

[0025] Examples of suitable viscosity index improvers includepolymethacrylates, copolymers of vinyl pyrrolidone and methacrylates,polybutenes, and styrene-acrylate copolymers.

[0026] Examples of suitable pour point and/or floc point depressantsinclude polymethacrylates such as methacrylateethylene-vinyl acetateterpolymers; alkylated naphthalene derivatives; and products ofFriedel-Crafts catalyzed condensation of urea with naphthalene orphenols.

[0027] Examples of suitable detergents and/or dispersants includepolybutenylsuccinic acid amides; polybutenyl phosphonic acidderivatives; long chain alkyl substituted aromatic sulfonic acids andtheir salts; and metal salts of alkyl sulfides, of alkyl phenols, and ofcondensation products of alkyl phenols and aldehydes.

[0028] Examples of suitable anti-foam agents include silicone polymersand some acrylates.

[0029] Examples of suitable anti-wear and extreme pressure resistanceagents include sulfurized fatty acids and fatty acid esters, such assulfurized octyl tallate; sulfurized terpenes; sulfurized olefins;organopolysulfides; organo phosphorus derivatives including aminephosphates, alkyl acid phosphates, dialkyl phosphates,aminedithiophosphates, trialkyl and triaryl phosphorothionates, trialkyland triaryl phosphines, and dialkylphosphites, e.g., amine salts ofphosphoric acid monohexyl ester, amine salts of dinonylnaphthalenesulfonate, triphenyl phosphate, trinaphthyl phosphate, diphenyl cresyland dicresyl phenyl phosphates, naphthyl diphenyl phosphate,triphenylphosphorothionate; dithiocarbamates, such as an antimonydialkyl dithiocarbamate; chlorinated and/or fluorinated hydrocarbons,and xanthates.

[0030] Under some conditions of operation, it is believed that thepresence in lubricants of the types of polyether polyols that have beenprominent constituents of most prior art lubricant base stocks taught asuseful with fluorocarbon refrigerant working fluids are less thanoptimally stable and/or inadequately compatible with some of the mostuseful lubricant additives. Thus, in one embodiment of this invention,it is preferred that the lubricant base stocks and lubricants besubstantially free of such polyether polyols. By “substantially free”,it is meant that the compositions contain no more than about 10% byweight, preferably no more than about 2.6% by weight, and morepreferably no more than about 1.2% by weight of the materials noted.

[0031] One major embodiment of the present invention is a refrigerantworking fluid comprising both a suitable heat transfer fluid such as afluorocarbon and a lubricant according to this invention. Preferably,the refrigerant working fluid and the lubricant should have chemicalcharacteristics and be present in such a proportion to each other thatthe working fluid remains homogeneous, i.e., free from visuallydetectable phase separations or turbidity, over the entire range ofworking temperatures to which the working fluid is exposed duringoperation of a refrigeration system in which the working fluid is used.This working range may vary from −60° C. to as much as +175° C. It isoften adequate if the working fluid remains single phase up to +30° C.,although it is increasingly more preferable if the single phase behavioris maintained up to 40, 56, 71, 88, or 100° C. Similarly, it is oftenadequate if the working fluid compositions remains a single phase whenchilled to 0° C., although it is increasingly more preferable if thesingle phase behavior persists to −10, −20, −30, −40, or −55° C. Singlephase mixtures with chlorine free hydrofluorocarbon refrigerant workingfluids are usually obtained with the suitable and preferred types ofesters described above.

[0032] Inasmuch as it is often difficult to predict exactly how muchlubricant will be mixed with the heat transfer fluid to form a workingfluid, it is most preferable if the lubricant composition forms a singlephase in all proportions with the heat transfer fluid over thetemperature ranges noted above. This however, is a very stringentrequirement, and it is often sufficient if there is single phasebehavior over the entire temperature range for a working fluid mixturecontaining up to 1% by weight of lubricant according to this invention.Single phase behavior over a temperature range for mixtures containingup to 2, 4, 10, and 15% by weight of lubricant is successively morepreferable.

[0033] In some cases, single phase behavior is not required. The term“miscible” is used in the refrigeration lubrication art and hereinafter,except when part of the phrase “miscible in all proportions”, when twophases are formed but are readily capable of being mixed into a uniformdispersion that remains stable as long as it is at least moderatelyagitated mechanically. Some refrigeration (and other) compressors aredesigned to operate satisfactorily with such miscible mixtures ofrefrigerant working fluid and lubricant. In contrast, mixtures that leadto coagulation or significant thickening and form two or more phases areunacceptable commercially and are designated herein as “immiscible”. Anysuch mixture described below is a comparative example and not anembodiment of the present invention.

[0034] Another major embodiment of the invention is the use of alubricant according to the invention, either as total lubricant orlubricant base stock, in a process of operating refrigerating machineryin such a manner that the lubricant is in contact with the refrigerantworking fluid.

[0035] The practice of the invention may be further understood andappreciated by consideration of the following examples and comparativeexamples.

[0036] General Ester Synthesis Procedure

[0037] The alcohol(s) and acid(s) to be reacted, together with asuitable catalyst such as dibutyltin diacetate, tin oxalate, phosphoricacid, and/or tetrabutyl titanate, were charged into a round bottomedflask equipped with a stirrer, thermometer, nitrogen sparging means,condenser, and a recycle trap. Acid(s) were charged in about a 15% molarexcess over the alcohol(s). The amount of catalyst was from 0.02 to 0.1%by weight of the weight of the total acid(s) and alcohol(s) reacted.

[0038] The reaction mixture was heated to a temperature between about220 and 230° C., and water from the resulting reaction was collected inthe trap while refluxing acids were returned to the reaction mixture.Partial vacuum was maintained above the reaction mixture as necessary toachieve a reflux rate of between 8 and 12% of the original reactionmixture volume per hour.

[0039] The reaction mixture was sampled occasionally for determinationof hydroxyl number, and after the hydroxyl number had fallen below 5.0mg of KOH per gram of mixture, the majority of the excess acid wasremoved by distillation after applying the highest vacuum obtainablewith the apparatus used, corresponding to a residual pressure of about0.05 torr, while maintaining the reaction temperature. The reactionmixture was then cooled, and any residual acidity was removed, ifdesired, by treatment with lime, sodium hydroxide, or epoxy esters. Theresulting lubricant or lubricant base stock was dried and filteredbefore phase compatibility testing.

[0040] General Procedure for Phase Compatibility Testing

[0041] One milliliter (“ml”) of the lubricant to be tested is placedinto a thermal shock resistant, volumetrically graduated glass test tube17 millimeters (“mm”) in diameter and 145 mm long. The test tube is thenstoppered and placed into a cooling bath regulated to −29±0.2° C. Afterthe tube and contents have equilibrated in the cooling bath for 5minutes (“min”), sufficient refrigerant working fluid is added to give atotal volume of 10 ml.

[0042] At least 15 min after the working fluid has been added, duringwhich time the tube and contents have been equilibrating in the coolingbath and the contents may have been agitated if desired, the tubecontents are visually examined for evidence of phase separation. Ifthere is any such phase separation, the tube is shaken to determinewhether the combination can be rated as miscible or is totallyunacceptable.

[0043] If there is no evidence of phase separation at −29° C., thetemperature of the cooling bath is usually lowered at a rate of 0.3° permin until phase separation is observed. The temperature of firstobservation of phase separation, if within the range of the coolingequipment used, is then noted as the insolubility onset temperature.

[0044] Composition of Specific Examples

[0045] A suitable ester mixture as described above was prepared byreacting a mixture of alcohol molecules in which 99.4% were PEmolecules, with most of the remainder being DPE molecules, with amixture of acid molecules that included 46.7% of pentanoic (=n-valeric)acid, 21.5% of 2-methylbutanoic acid, and 31.6% of3,5,5-trimethylhexanoic acid, with the remainder predominantly otherbranched C₉ monobasic acids. This ester mixture had an ISO grade of 32.

[0046] A second suitable ester mixture as described above was preparedby reacting a mixture of alcohol molecules in which 99.4% were PEmolecules, with most of the remainder being DPE molecules, with amixture of acid molecules that included 66.8% of pentanoic (=n-valeric)acid, 28.4% of 2-methylbutanoic acid, and 4.6% of adipic acid, with theremainder being predominantly 3-methylbutanoic acid. This lubricant basestock had an ISO grade of 32.

The invention claimed is:
 1. A composition of matter suitable forserving as a lubricant or lubricant base stock, said composition being aliquid with a viscosity between about 22.5 and about 44 centistokes at40° C. and consisting essentially of a mixture of polyol ester moleculesin which at least about 92% of the alcohol moieties are selected fromthe group consisting of the alcohol moieties derived from TMP, DTMP, PE,and DPE and at least about 92% of the acyl groups are selected from thegroup consisting of the acyl groups of all the straight and branchedchain monobasic and dibasic carboxylic acids with from four to twelvecarbon atoms each, said alcohol moieties and acyl groups being furtherselected subject to the constraints that (a) a total of at least about3% of the acyl groups in the mixture are acyl groups of i-C₅ acid; (b)the ratio of the % of acyl groups in the mixture that contain 8 or morecarbon atoms and are unbranched to the % of acyl groups in the mixturethat are both branched and contain not more than six carbon atoms is notgreater than about 1.56; (c) the % of acyl groups in the mixture thatcontain at least nine carbon atoms, whether branched or not, is notgreater than about 81; and (d) not more than about 2% of the acyl groupsin the ester mixture are part of acid molecules with more than twocarboxyl groups each; and either (d)(1) a total of at least about 20% ofthe acid molecules in the mixture are one of the trimethylhexanoicacids; at least about 90% of the alcohol moieties in the esters arethose of PE; and not more than about 7.5% of the acyl groups in theester mixture are dibasic; or (d)(2) at least about 2.0%, but not morethan about 13%, of the acyl groups in the ester mixture are dibasic; anda total of at least about 82% of the monobasic acyl groups in the acidmixture have either five or six carbon atoms each.
 2. A compositionaccording to claim 1, with a viscosity between about 25.0 and about 38.5centistokes at 40° C. and consisting essentially of a mixture of polyolester molecules in which the alcohol moieties and acyl groups areselected subject to the constraints that (a) a total of at least about7% of the acyl groups in the mixture are acyl groups of i-C₅ acid; (b)the ratio of the % of acyl groups in the mixture that contain 8 or morecarbon atoms and are unbranched to the % of acyl groups in the mixturethat are both branched and contain not more than six carbon atoms is notgreater than about 1.56; (c) the % of acyl groups in the mixture thatcontain at least nine carbon atoms, whether branched or not, is notgreater than about 67; and (d) either (d)(1) a total of at least about29% of the acyl groups in the mixture are in one of thetrimethylhexanoic acids and not more than about 6.0% of the acyl groupsin the acid mixture are dibasic; or (d)(2) at least about 2.8%, but notmore than about 10%, of the acyl groups in the ester mixture are dibasicand a total of at least about 85% of the monobasic acyl groups in theester mixture have either five or six carbon atoms each.
 3. Acomposition according to claim 2 with a viscosity between about 27.4 andabout 36.6 centistokes at 40° C. and consisting essentially of a mixtureof polyol ester molecules in which at least about 95% of the alcoholmoieties are selected from the group consisting of the alcohol moietiesderived from TMP, DTMP, PE, and DPE and at least about 95% of the acylgroups are selected from the group consisting of the acyl groups of allthe straight and branched chain monobasic and dibasic carboxylic acidswith from four to twelve carbon atoms each, said alcohol moieties andacyl groups being further selected subject to the constraints that (a) atotal of at least about 10% of the acyl groups in the mixture are acylgroups of i-C₅ acid; (b) the ratio of the % of acyl groups in themixture that contain 8 or more carbon atoms and are unbranched to the %of acyl groups in the mixture that are both branched and contain notmore than six carbon atoms is not greater than about 1.21; (c) the % ofacyl groups in the mixture that contain at least nine carbon atoms,whether branched or not, is not greater than about 67; and (d) not morethan 1% of the acyl groups in the esters contain more than two carboxylgroups each; and either (d)(1) a total of at least about 35% of the acidmolecules in the mixture are one of the trimethylhexanoic acids and notmore than about 3% of the acyl groups in the ester mixture are dibasic;or (d)(2) at least about 3.6% of the acyl groups in the ester mixtureare dibasic; and a total of at least about 93% of the monobasic acylgroups in the acid mixture have five carbon atoms each.
 4. A compositionaccording to claim 3 with a viscosity between about 28.5 and about 36.6centistokes at 40° C. and consisting essentially of a mixture of polyolester molecules in which the alcohol moieties and acyl groups areselected subject to the constraints that (a) a total of at least about14% of the acyl groups in the mixture are acyl groups of i-C₅ acid; (b)the ratio of the % of acyl groups in the mixture that contain 8 or morecarbon atoms and are unbranched to the % of acyl groups in the mixturethat are both branched and contain not more than six carbon atoms is notgreater than 1.00; (c) the % of acyl groups in the mixture that containat least nine carbon atoms, whether branched or not, is not greater thanabout 49; and (d) not more than 0.4% of the acyl groups in the estermixture contain more than two carboxyl groups each; and either (d)(1) atotal of at least about 35% of the acyl groups in the mixture are one ofthe trimethylhexanoic acids and at least about 29% of the acyl groups inthe mixture are 3,5,5-trimethylhexanoyl groups; at least about 95% ofthe alcohol moieties in the esters are those of PE; and not more thanabout 1.7% of the acyl groups in the ester mixture are dibasic; or(d)(2) at least about 4.1% of the acyl groups in the ester mixture aredibasic and contain from five to seven carbon atoms each and a total ofat least about 96% of the monobasic acyl groups in the acid mixture havefive carbon atoms each.
 5. A composition according to claim 4 with aviscosity between about 29.4 and about 35.7 centistokes at 40° C. andconsisting essentially of a mixture of polyol ester molecules in whichat least about 95% of the acyl groups are selected from the groupconsisting of the acyl groups of all the straight and branched chainmonobasic and dibasic carboxylic acids with from four to twelve carbonatoms each and the alcohol moieties and acyl groups are selected subjectto the constraints that (a) a total of at least 16% of the acyl groupsin the ester mixture are acyl groups of i-C₅ acid; (b) the ratio of the% of acyl groups in the mixture that contain 8 or more carbon atoms andare unbranched to the % of acyl groups in the mixture that are bothbranched and contain not more than five carbon atoms is not greater thanabout 1.00; (c) the % of acyl groups in the mixture that contain atleast nine carbon atoms, whether branched or not, is not greater thanabout 49; and (d) at least about 95% of the alcohol moieties in theesters are those of PE, and either (d)(1) a total of at least about 41%of the acyl groups in the ester mixture are acyl groups of one of thetrimethylhexanoic acids and at least about 29% of the acyl groups in themixture are 3,5,5-trimethylhexanoyl groups; and not more than 1% of theacyl groups in the ester mixture are dibasic; or (d)(2) at least 4.9% ofthe acyl groups in the ester mixture are dibasic.
 6. A compoundedlubricant consisting essentially of at least about 95% by weight of acomposition according to claim 5 and a balance of one or more additivesselected from the group consisting of oxidation resistance and thermalstability improvers, corrosion inhibitors, metal deactivators, lubricityadditives, viscosity index improvers, pour and floc point depressants,detergents, dispersants, antifoaming agents, anti-wear agents, andextreme pressure resistant additives.
 7. A compounded lubricantconsisting essentially of at least about 92% by weight of a compositionaccording to claim 3 and a balance of one or more additives selectedfrom the group consisting of oxidation resistance and thermal stabilityimprovers, corrosion inhibitors, metal deactivators, lubricityadditives, viscosity index improvers, pour and floc point depressants,detergents, dispersants, antifoaming agents, anti-wear agents, andextreme pressure resistant additives.
 8. A compounded lubricantconsisting essentially of at least about 92% by weight of a compositionaccording to claim 2 and a balance of one or more additives selectedfrom the group consisting of oxidation resistance and thermal stabilityimprovers, corrosion inhibitors, metal deactivators, lubricityadditives, viscosity index improvers, pour and floc point depressants,detergents, dispersants, antifoaming agents, anti-wear agents, andextreme pressure resistant additives.
 9. A compounded lubricantconsisting essentially of at least about 92% by weight of a compositionaccording to claim 1 and a balance of one or more additives selectedfrom the group consisting of oxidation resistance and thermal stabilityimprovers, corrosion inhibitors, metal deactivators, lubricityadditives, viscosity index improvers, pour and floc point depressants,detergents, dispersants, antifoaming agents, anti-wear agents, andextreme pressure resistant additives.
 10. A refrigerant working fluidconsisting essentially of at least 85% by weight of a primary heattransfer fluid selected from the group consisting of pentafluoroethane,1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroethanes, andmixtures thereof and a balance of a lubricant according to claim
 9. 11.A refrigerant working fluid consisting essentially of at least 85% byweight of a primary heat transfer fluid selected from the groupconsisting of pentafluoroethane, 1,1-difluoroethane,1,1,1-trifluroethane, tetrafluoroethanes, and mixtures thereof and abalance of a lubricant according to claim
 8. 12. A refrigerant workingfluid consisting essentially of at least 85% by weight of a primary heattransfer fluid selected from the group consisting of pentafluoroethane,1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroethanes, andmixtures thereof and a balance of a lubricant according to claim
 7. 13.A refrigerant working fluid consisting essentially of at least 85% byweight of 1,1,1,2-tetrafluroethane and a balance of a lubricantaccording to claim
 6. 14. A refrigerant working fluid consistingessentially of at least 85% by weight of a primary heat transfer fluidselected from the group consisting of pentafluoroethane,1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroethanes, andmixtures thereof and a balance of a lubricant according to claim
 5. 15.A refrigerant working fluid consisting essentially of at least 85% byweight of a primary heat transfer fluid selected from the groupconsisting of pentafluoroethane, 1,1-difluoroethane,1,1,1-trifluroethane, tetrafluoroethanes, and mixtures thereof and abalance of a lubricant according to claim
 4. 16. A refrigerant workingfluid consisting essentially of at least 85% by weight of a primary heattransfer fluid selected from the group consisting of pentafluoroethane,1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroethanes, andmixtures thereof and a balance of a lubricant according to claim
 3. 17.A refrigerant working fluid consisting essentially of at least 85% byweight of a primary heat transfer fluid selected from the groupconsisting of pentafluoroethane, 1,1-difluoroethane,1,1,1-trifluroethane, tetrafluoroethanes, and mixtures thereof and abalance of a lubricant according to claim
 2. 18. A refrigerant workingfluid consisting essentially of at least 85% by weight of a primary heattransfer fluid selected from the group consisting of pentafluoroethane,1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroethanes, andmixtures thereof and a balance of a lubricant according to claim
 1. 19.A process for operating a refrigerator comprising cyclic compression,liquefaction, expansion, and evaporation of a heat transfer fluid, saidheat transfer fluid consisting essentially of at least 85% by weight of1,1,1,2-tetrafluoroethane and a balance of a lubricant according toclaim
 6. 20. A process for operating a refrigerator comprising cycliccompression, liquefaction, expansion, and evaporation of a heat transferfluid, said heat transfer fluid consisting essentially of at least 85%by weight of a primary heat transfer fluid selected from the groupconsisting of pentafluoroethane, 1,1-difluoroethane,1,1,1-trifluroethane, tetrafluoroethanes, and mixtures thereof and abalance of a lubricant according to claim 1.